Glutamate is the principal excitatory amino acid neurotransmitter in the central nervous system (CNS) and activates two distinct families of glutamate receptors (GluRs). The ionotropic glutamate receptor (iGluR) family is composed of NMDA (N-methyI-D-aspartate), AMPA (alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid), and kainate receptors that mediate fast synaptic transmission, while modulation of cell excitability and synaptic transmission is mediated by the family of glutamate activated G protein-coupled receptors (GPCRs), the metabotropic glutamate receptors (mGluRs). Molecular cloning of mGluRs revealed eight distinct receptor subtypes: Group I mGluRs (mGluRs 1 and 5) couple to the Gq signaling pathway, whereas group II (mGluRs 2 and 3) and group III (mGluRs 4, 6, 7 and 8) mGluRs couple to Gi/Go signaling pathways in heterologous expression systems. Recent studies on protein-protein interactions combine coimmunoprecipitation (colP) with mass spectrometry (MS) to generate a catalog of putative interacting proteins that are tested to validate or negate their interaction with the target protein. Thus the goal of this study is to identify proteins that interact with the group I receptor mGluR5a using a proteomics approach. Group I mGluR agonists vary widely and they can either amplify or attenuate excitotoxic cell death. Three variables affect group I mGluR responses to agonists: 1) the heteromeric composition of NMDA receptors in the cell, 2) the time of exposure to drugs, and 3) the presence of astrocytes (see Nicoletti et al., 1999). We submit that cell-specific mGluR-protein interactions may also account, in part, for the dual response of group I mGluRs to agonists. The extracellular amino terminal domain (ATD) of mGluRs is approximately 500 amino acids, or one half of the protein mass of an individual mGluR. Outside of the glutamate-binding site, a functional role for the large extracellular ATD has not been described however the ATD may serve to interact with proteins in the synaptic cleft or the extracellular matrix. If ATD-binding proteins are identified, they may provide alternative mechanisms for the regulation of mGluR signaling in the CNS. Our preliminary studies have identified known, as well as novel, mGluR5a-interacting proteins, including a protein that regulates mGluR5a signaling. These data provide proof of concept that colP/MS can identify novel mGluR5a interacting proteins, that these proteins may shed light on the molecular mechanisms that contribute to the diverse roles of mGluR5a in the CNS, and that these interactions may be overlooked using more traditional protein-protein interaction studies.